The answer to 'roof decking water stains' is that water stains on the roof decking are typically caused by roof leaks or condensation. The symptoms include discolored, warped, or soft wood decking. The solutions depend on the underlying cause, but may involve repairing roof leaks, improving ventilation, or replacing damaged decking.
If the stains are limited and the decking is still structurally sound, you may be able to address the issue with some DIY repairs. However, if the damage is extensive or the root cause is unclear, it's best to call a professional roofing contractor to diagnose and fix the problem properly.